It has become common to treat many diseases using implantable medical devices (IMDS) that are chronically implanted within the body of a patient. Examples of such medical devices include pacemakers, defibrillators, drug-delivery devices, and electro-stimulators for stimulating nerves, muscles, and other tissue.
One problem associated with the chronic implantation of IMDs involves the growth of fibrous tissue around the device. When a foreign object such as an IMD is introduced into a patient's body, the body's auto-immune system forms a collagen capsule around the foreign object. This capsule, which has fibrous tissue, attaches to the IMD in a manner that prevents easy extraction of the device. This makes it difficult to replace or re-locate a medical device after it has been in the body any significant amount of time. This problem is particularly prevalent when dealing with implantable medical leads.
Implantable medical leads have many uses. For example, leads carrying electrodes and other sensors are often positioned within a chamber of the heart or in the associated vasculature. These leads, which are coupled at one end to an IMD, may be used to delivery electrical stimulation to cardiac tissue, and/or to sense physiological signals. Leads may also be utilized to deliver medication to the body as controlled by a drug delivery device.
As noted above, the formation of fibrous tissue surrounding a medical lead results in problems when the lead is to be replaced or re-located. The problems are exasperated by the formation of small micro cracks in the surface of the lead body. These cracks result when leukocytes release oxygen-free radicals causing an autoxidation reaction at the lead's surface. The small crevices create additional surface area and spaces within which fibrous tissue can bond, making chronic lead extraction even more difficult.
Many methods have been devised in attempt to prevent the bonding of collagenous capsule tissue to the surface of implantable leads. If such bonding could be prevented, the extraction of chronically-implanted leads would be greatly simplified. One manner of attempting to prevent tissue in-growth is disclosed in U.S. Pat. No. 5,609,622, which describes coating a lead with a porous Polytetrafluoroethylene (PTFE) layer such as may be formed of expanded PTFE (e-PTFE), and which has a pore size of less than 10 microns or smaller so that the pore size is very small, and tissue in-growth is prevented.
Other methods of preventing tissue in-growth are directed more specifically at eliminating the formation of tissue around the electrode structures carried on some lead bodies. One mechanism disclosed in U.S. Pat. No. 4,934,049 issued to Kiekhafer et al. involves injecting silicone rubber into the spaces between the individual coils of an electrode structure. The resulting thin coating of silicone rubber surrounding the exterior of the electrode coils minimizes tissue in-growth between the filars of the coils, while leaving a portion of the coils exposed to deliver electrical stimulation to a patient.
Another approach to preventing tissue in-growth is disclosed in U.S. Pat. No. 5,090,422, which describes the use of biocompatible porous materials such as woven, porous polyurethane and porous polytetrafluoroethylene that may be used to cover an electrode or lead surface. The material is insulative when dry, but becomes conductive when bodily fluids penetrate the pores of the material. The porous covering has an adequately small pore size and fibril length to preclude substantial tissue in-growth
Although the foregoing mechanisms have been developed in attempt to prevent collagen formation with the surface of an IMD, problems still remain. The foregoing mechanisms are not entirely effective in preventing collagen attachment within the small micro cracks formed in the surface of leads, for example. Moreover, the foregoing mechanisms does not address the problems associated with autoxidation caused by the oxygen-free radicals. Therefore, what is needed is an improved system and method to prevent tissue in-growth in the surface of a chronically-implanted medical device.